1. Field of the Invention
The present invention relates to an optical transmission apparatus in which a laser diode (hereinafter called LD) is used as a luminous element for outputting an optical data signal.
2. Description of the Prior Art
FIG. 1 illustrates a circuit embodiment of an optical transmission apparatus according to the prior art which has been described for example in an article No. 2246 title "Optical Transmission Apparatus with 100 Mb/s Laser Diode Provided with Carrier Detection Circuit" reported at the General National Assembly of Society of Electronic Communication 1981. In FIG. 1, numeral 1 designates a transmission data input terminal to which transmission data is input, numeral 2 a modulator adapted to modulate a current to a binary current in accordance with the transmission data input to the input terminal, numeral 3 an LD as a luminous element which emits light in accordance with the binary current output from the modulator 2, numeral 4 a light receiving element adapted to receive a portion of the light emitted by the LD 3 and transduce, or convert the received optical signal to an electrical signal, numeral 5 a reference current source adapted to output a reference current, numeral 6 a current amplifier adapted to supply a drive bias current which is proportional to the difference between the reference current provided by the reference current source 5 and the signal current transduced by the light receiving element 4, numeral 7 a bias power source for the light receiving element 4 and numeral 8 a capacitor connected in parallel with the light receiving element 4.
FIG. 2 is a characteristic diagram illustrating P-I characteristics of the drive current for an LD vs the optical output of the LD. In FIG. 2, symbol A designates the P-I characteristic at a low temperature while symbol B designates the P-I characteristic at a high temperature.
FIG. 3 illustrates an example of an embodiment of the current amplifier 6 wherein numerals 61 through 63 designate transistors.
Operation of the prior optical transmission apparatus as described above will next be explained. The transmission data input to the input terminal 1 is input to the modulator 2. The modulator 2 generates a current corresponding to the transmission data and supplies it to the LD 3. The transmission data is also input to the reference current source 5 and the reference current I.sub.O which is obtained by adding a current proportional to the mark ratio of the transmission data and a constant current will be output therefrom. The portion of the optical output from the LD 3 is led to the light receiving element 4, so that the light receiving element current I.sub.PD flows. The differential current between the reference current I.sub.O from the source 5 and the current I.sub.PD flowing through the element 4 is input to the current amplifier 6 and after being amplified to a specified magnification .beta., it is supplied to the LD 3 as the bias current. The bias current always flows through the LD 3, and the LD illuminates when the current from the modulator 2 flows through the LD 3. As the optical output from the LD 3 becomes stronger, the current flowing through the light receiving element 4 will increase. Therefore the output current from the current amplifier 6 will be smaller whereby the optical output from the LD 3 will be reduced. In the case where that the optical output from the LD 3 is reduced, the optical output from LD 3 will be conversely increased due to a similar reason. Accordingly, the optical output may be substantially kept at a constant value owing to the negative feedback operation as described above.
As far as the peak value of the optical signal at the time of transmission of the digital signal is concerned, the peak value P.sub.out of the optical output from the LD 3 may be expressed in the following equation: EQU P.sub.out =A(I.sub.B +I.sub.OP -I.sub.th) (1)
provided, EQU I.sub.B =.beta.(I.sub.O -I.sub.PD) (2) EQU I.sub.PD =mDLP.sub.out ( 3)
where
I.sub.B =bias current (current output from the current amplifier) PA1 I.sub.OP =modulated current (current output from the modulator) PA1 I.sub.th =LD threshold current PA1 .beta.=amplification ratio of the current amplifier PA1 I.sub.PD =light receiving element current PA1 I.sub.O =reference current PA1 m=mark ratio of the digital signal (0&lt;m.ltoreq.1) PA1 L=current transducing ratio of optical output vs light receiving element PA1 A=LD current/optical transducing efficient PA1 D=pulse duty factor.
From the foregoing equations (1), (2) and (3), the following equation may be derived: ##EQU1##
In the equation (4), in order that the optical output P.sub.out be constant regardless of mark ratio change M, the reference current I.sub.O must be controlled in accordance with the mark ratio. And supposing P.sub.out =K (constant), EQU I.sub.O =I.sub.O1 +mI.sub.O2
provided, ##EQU2## EQU I.sub.O2 =KDL
As expressed by the foregoing equation (5), the reference current source 5 will supply the current I.sub.O comprising a constant current I.sub.O1 and the current mI.sub.O2 proportional to the mark ratio, so that the optical output from the LD 3 may be constant regardless of the mark ratio.
Operation of the optical transmission apparatus when the temperature has varied will next be explained. Supposing that the parameter which varies depending on the temperature is only the LD threshold current I.sub.th as illustrated in FIG. 2, the following equation may be derived from equation (4): ##EQU3##
If the value of the Fabry-Perot type LD of 1.3 .mu.m band is considered, values of the efficient A, current transducing ratio L and .differential.I.sub.th /.differential.t are almost as follows: EQU A=0.11(W/A) EQU L=0.14(A/W) ##EQU4## Then supposing that the temperature range is -30.degree. C.-+85.degree. C., and the case of the optical output 1 mW at T=25.degree. C. is considered. In order that the optical output be less than 1 db with the respective mark ratio, the following equation may be derived from equation (6): ##EQU5## From equation (7), .beta..gtoreq.17737 is produced, provided that the minimum value of the mark ratio is 1/8.
As explained above, a very large value of the current amplification ratio .beta. of the current amplifier 6 is required, therefore such a Darlington type amplifier as shown in FIG. 3 is employed. Supposing that the current amplification ratios of the transistors 61 through 63 are respectively .beta..sub.1, .beta..sub.2 and .beta..sub.3, the current amplification ratio of the amplifier will be expressed as follows: EQU .beta.=.beta..sub.1 .multidot..beta..sub.2 .multidot..beta..sub.3( 8)
In general, since a current amplification ratio of an npn transistor which may be obtained is more than 30, the current amplification ratio .beta. obtained by the current amplifier shown in FIG. 3 is more than 27,000. Accordingly, if the transistors 61 through 63 are in conductive condition, then the current amplifier 6 shown in FIG. 3 is supposed to have a sufficiently large current amplification ratio, so that an adequate APC (Automatic Power Control) characteristics may be obtained.
Since the optical transmission apparatus according to the prior art is constituted as above explained, the bias current I.sub.B of the LD 3 is generally set below the threshold current I.sub.th. Accordingly, at a high temperature, even if the bias current I.sub.B1 is made large enough as seen in FIG. 2, the bias current I.sub.B2 may be almost zero at a low temperature as seen in FIG. 2. In this low temperature condition, the transistors 61-63 of the current amplifier shown in FIG. 3 are shut down, and the current amplification ratio of the amplifier will become so small that the amplification ratio required for keeping the peak value of the optical output constant relative to the mark ratio and the temperature fluctuation will no longer be secured and this reduction of the current amplification ratio will degrade the APC characteristics.
Further, it is sometimes necessary to stop the optical output from the optical transmission apparatus in order to check interruption of a network by passing another optical signal through an optical fiber for the purpose of maintenance of an optical transmission apparatus.
It also happens sometimes that the bias current flowing through the LD 3 will be monitored for maintenance of the apparatus and when the bias current value becomes abnormal, it is detected to energize an alarm.
In FIG. 4, there is illustrated an optical transmission apparatus according to the prior art which is provided with the function of stopping any optical output from the optical transmission apparatus and monitoring the bias current of the LD 3 as above-described.
In FIG. 4, the circuits having the similar functions as those of the circuits shown in FIG. 1 are denoted by the same numerals. It is to be noted that the transmission apparatus shown in FIG. 4 executes transmission operation substantially in a similar manner as that shown in FIG. 1. In the apparatus shown in FIG. 4, the modulation circuit 2 comprises transistors 21 and 22 forming a current switching circuit, a constant current source 23 and a switch 24 and is adapted to draw a pulse current, or binary current into the collector of the transistor 22 in response to the input data to the base of the transistor 21 whereby the LD 3 generates the optical output in response to the pulse current by the aid of the bias current from the amplifier 6. The reference current source 5 comprises a controllable constant current source 51 and a switch 52. The switches 24 and 52 are turned off by an inhibit signal supplied to an inhibit control terminal 9 in order to stop the optical output from the optical transmission apparatus.
The apparatus shown in FIG. 4 is further provided with a monitoring circuit 10 for monitoring the bias current. The monitoring circuit 10 comprises a resistor 101 for monitoring current connected between the cathode of the LD 3 and the output stage transistor of the current amplifier 6 and a buffer amplifier having a unitary gain which comprises resistors 102, 103, 104 and 105 and an operational amplifier 106. The monitoring circuit 10 is adapted to generate the same voltage as that generated across the opposite ends of the resistor 101 to the output terminal 107 whereby the current flowing through the LD 3 and the amplifier 6 can be monitored.
According to the apparatus shown in FIG. 4, when the optical output is prohibited by the inhibit signal from the terminal 9, the potential between the opposite ends of the resistor 101 in the monitoring circuit 10 will be zero and the voltage between the anode and cathode terminals of the LD 3 will also be zero. Accordingly the common mode input voltage to the buffer amplifier will be equal to the potential at the anode of the LD 3 whereby it will be equal to the potential of a power source V.sub.CC connected to the anode of the LD. Under this condition, the input voltage to the buffer amplifier will be outside the common mode operational range, and therefore the output terminal 107 may be dropped to the potential of a power source V.sub.EE connected to the operational amplifier 106. This may cause, therefore, the monitoring circuit not to operate properly and provide an erroneous alarm when the optical output is prohibited.